Linux From Scratch - Version 20150723-systemd

Chapter 6. Installing Basic System Software

6.2. Preparing
Virtual Kernel File Systems

Various file systems exported by the kernel are used to communicate
to and from the kernel itself. These file systems are virtual in that
no disk space is used for them. The content of the file systems
resides in memory.

Begin by creating directories onto which the file systems will be
mounted:

mkdir -pv $LFS/{dev,proc,sys,run}

6.2.1. Creating Initial Device Nodes

When the kernel boots the system, it requires the presence of a few
device nodes, in particular the console and null
devices. The device nodes must be created on the hard disk so that
they are available before udevd has been started, and
additionally when Linux is started with init=/bin/bash. Create the devices by
running the following commands:

mknod -m 600 $LFS/dev/console c 5 1
mknod -m 666 $LFS/dev/null c 1 3

6.2.2.
Mounting and Populating /dev

The recommended method of populating the /dev directory with devices is to mount a virtual
filesystem (such as tmpfs) on the
/dev directory, and allow the devices
to be created dynamically on that virtual filesystem as they are
detected or accessed. Device creation is generally done during the
boot process by Udev. Since this new system does not yet have Udev
and has not yet been booted, it is necessary to mount and populate
/dev manually. This is accomplished
by bind mounting the host system's /dev directory. A bind mount is a special type of
mount that allows you to create a mirror of a directory or mount
point to some other location. Use the following command to achieve
this:

This ensures that all devpts-created device nodes are owned
by group ID 5. This is the ID we will use later on for the
tty group. We use the group
ID instead of a name, since the host system might use a
different ID for its tty
group.

mode=0620

This ensures that all devpts-created device nodes have mode
0620 (user readable and writable, group writable). Together
with the option above, this ensures that devpts will create
device nodes that meet the requirements of grantpt(), meaning
the Glibc pt_chown helper binary
(which is not installed by default) is not necessary.

In some host systems, /dev/shm is a
symbolic link to /run/shm. The /run
tmpfs was mounted above so in this case only a directory needs to
be created.